S-(Methylthio)-homocysteine (SMETH) is disclosed in Japanese Patent Publication Kokai No. 77 83710, Jul. 12, 1977 (C.A. 88:23393m). This publication, however, fails to recognize the potential therapeutic effects of SMETH, particularly its ability to function as a potent glutamine antagonist in cancer cells. Other glutamine antagonists which are effective as anti-cancer agents are natural antibiotics. These particularly include 6-diazo-5-oxo-norleucine, azaserine, and acivicin. These antibiotic compounds tend to be disadvantageous because they exhibit a low degree of biochemical specificity.